Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839

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Ruthenium(II) bipyridine complexes bearing new keto–enol azoimine ligands: Synthesis, structure, electrochemistry and DFT calculations Mousa Al-Noaimi a,⇑, Firas F. Awwadi b, Ahmad Mansi a, Obadah S. Abdel-Rahman c, Ayman Hammoudeh d, Ismail Warad e a

Department of Chemistry, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan Department of Chemistry, The University of Jordan, Amman 11942, Jordan Fachbereich Chemie der Universität Konstanz, Universitätstraße 10, D-78457 Konstanz, Germany d Chemistry Department, Yarmouk University, P.O. Box 566, Irbid, Jordan e Department of Chemistry, AN-Najah National University, Nablus, Occupied Palestinian Territory b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 New azoimine-ligand (H2L) with a

The novel amiderazone ligand, PhANHAN@C(COCH3)ANHAPh(C„CH) (H2L), ligand was synthesized. The molecular structure of the ligand was determined by X-ray crystallography. Reaction of the ligand H2L with RuCl33H2O in ethanol at reflux temperature resulted in catalytic hydration of terminal acetylene group (C„CH) via Markovnikov selectivity to ketone (L1) and enol(L2). Mixed-ligand ruthenium complexes having a general formula, trans-[Ru(bpy)(Y)Cl2] (1–2) (where Y = L1 (1) and Y = L2 (2), bpy is 2.20 -bipyrdine) were achieved by the stepwise addition of equimolar amounts of (H2L) and bpy ligands. Theses complexes were characterized by spectroscopic (IR, UV–Vis, and NMR). The crystal structure of complex 1 showed that bidentate ligand L1 coordinates to Ru(II) by azo- and imine-nitrogen donor atoms. The electronic spectra of 1 and 1+ in dichloromethane have been modeled by time-dependent density functional theory (TD-DFT).

terminal acetylene was synthesized.  Markovnikov hydration of the terminal acetylene in H2L to ketone (L1) and enol (L2).  Two mixed-ligand Ru(II) complexes with L1 and L2.  The absorption spectrum of 1 and 1+ were modeled by TD-DFT.

O N

N

N Cl N O N

N H

N H

Cl

Ru

RuCl3

N

+

bpy

O

N N Cl

i n f o

Article history: Received 16 June 2014 Received in revised form 12 July 2014 Accepted 28 July 2014 Available online 7 August 2014 Keywords: Ruthenium Catalytic hydration Keto–enol tautomers Spectroelectrochemistry

Cl

H

H OH

N

a b s t r a c t The novel azoimine ligand, PhANHAN@C(COCH3)ANHPh(C„CH) (H2L), was synthesized and its molecular structure was determined by X-ray crystallography. Catalytic hydration of the terminal acetylene of H2L in the presence of RuCl33H2O in ethanol at reflux temperature yielded a ketone (L1 = PhAN@NAC(COCH3)@NAPh(COCH3) and an enol (L2 = PhAN@NAC(COCH3)@NAPhC(OH)@CH2) by Markovnikov addition of water. Two mixed-ligand ruthenium complexes having general formula, trans-[Ru(bpy)(Y)Cl2] (1–2) (where Y = L1 (1) and Y = L2 (2), bpy is 2.20 -bipyrdine) were achieved by the stepwise addition of equimolar amounts of (H2L) and bpy ligands to RuCl33H2O in absolute ethanol. Theses complexes were characterized by elemental analyses and spectroscopic (IR, UV–Vis, and NMR (1D 1 H NMR, 13C NMR, (DEPT-135), (DEPT-90), 2D 1H–1H and 13C–1H correlation (HMQC) spectroscopy)). The two complexes exhibit a quasi-reversible one electron Ru(II)/Ru(III) oxidation couple at 604 mV vs.

⇑ Corresponding author. Fax: +962 (5) 3826613. E-mail address: [email protected] (M. Al-Noaimi). http://dx.doi.org/10.1016/j.saa.2014.07.080 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

N Ru

N

a r t i c l e

O

M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839 Electrochemistry DFT calculation

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ferrocene/ferrocenium (Cp2Fe0/+) couple along with one electron ligand reduction at 1010 mV. The crystal structure of complex 1 showed that the bidentate ligand L1 coordinates to Ru(II) by the azo- and imine-nitrogen donor atoms. The complex adopts a distorted trans octahedral coordination geometry of chloride ligands. The electronic spectra of 1 and 1+ in dichloromethane have been modeled by timedependent density functional theory (TD-DFT). Ó 2014 Elsevier B.V. All rights reserved.

Introduction

Experimental

The hydration of alkynes to give carbonyl compounds is a well-known reaction that can be promoted by transition metal complexes [1–5]. The hydration of terminal alkynes gives either a methyl ketone (Markovnikov addition) or an aldehyde (antiMarkovnikov addition), whereas non-symmetrical internal alkynes gives two regioisomeric ketones. Traditional mercury (II) catalysts hydrate terminal alkynes with Markovnikov selectivity to methyl ketones [6–8]. The reaction is one of the earliest examples of homogeneous metal-complex catalysis [6]. Extensive research has been devoted to find catalysts based on less toxic metals, the most promising being gold(I), gold(III), platinum(II), and palladium(II). It is known that the addition of water to terminal alkynes using Hg(II), Au(III) [9,10], Ru(III) [5], Pt(II) [11], Pt(IV) [12], Cu(I) [13,14], Rh(III) [15,16] and other metal catalysts exclusively give methyl ketones. Catalytic anti-Markovnikov hydration of terminal alkynes to aldehydes was catalyzed by RuCl2/phosphine mixture [17]. In 1998, Tokunaga and Wakatsuki described the first anti-Markovnikov hydration catalyzed by ruthenium(II) complexes, which yielded aldehydes from terminal alkynes [18]. Transition metal complexes of a-diimine ligands (AN@CAC@NA) such as 2,20 -bipyridine (bpy) have attracted much attention for a long time [19–21] due to rich redox chemistry [22,23] and excellent photophysical and photochemical activities [24,25]. The replacement of one of the carbons in the diimine group with the more electronegative nitrogen atom give p-acidic an azoimine functional (AN@NAC@NA) ligand. These ligands are p-acceptors and can be used to stabilize low valent metal oxidation states [26–28]. Recently, we have adopted a simple technique to synthesize bidentate azoimine ligands (T) (where T is (Y) PhANAN@C(R)ANAPh(X), X = CH3, NO2, Cl, Br) by reacting hydrazonyl chloride with an appropriate aniline derivative [26–32]. These ligands are isoelectronic with the a-diimine group and by changing the substituents R and the substituent on the phenyl rings (Ph), the electronic properties of these ligands can be modified significantly. As part of our continuing interest in ruthenium azoimine chemistry, in this study we have synthesized a new PhANHAN@C(COCH3)ANHAPh(C„CH) ((H2L)) ligand. The terminal acetylene group (C„CH) of the ligand H2L was catalytically hydrated via Markovnikov addition of water to an enol ((C(OH)@CH2) and acetyl (COCH3) by refluxing the ligand (H2L) and RuCl33H2O in absolute ethanol. Also, two mixed-ligand ruthenium complexes having a general formula, trans-[Ru(bpy)(Y)Cl2] (1–2) (where Y is L1 = acetyl tautomer (1) and L2 = enol tautomer (2)) have been prepared by adding equimolar amount of 2,20 -bipyridine to the reaction mixture after 1 h. The presence of the two tautomers was confirmed by, the distortionless enhancement by polarization transfer (DEPT-135), (DEPT-90), 13C–1H correlation spectroscopy (HMQC) NMR techniques and X-ray for complex 1. The absorption spectra of complexes 1 and 1+ in dichloromethane have been modeled by time-dependent density functional theory (TD-DFT).

Materials All aniline derivatives, ruthenium trichloride trihydrate and solvents were purchased from Aldrich. Tetrabutylammonium hexafluorophosphate (TBAH), purchased from Aldrich, was recrystallized twice from 1:1 ethanol: water and then vacuum dried at 110 °C. (1Z)-N-(4-ethynylphenyl)-2-oxo-N0 -phenylpropanehydrazonamide (H2L) A solution of 1-(phenylhydrazono)-2-propanone (2.0 g, 10 mmol), 4-ethynylaniline (1.4 g, 12 mmol), and triethylamine (1.212 g, 12 mmol) in ethanol (10 mL) was refluxed for 2 h, and then the solvent was partially removed under reduced pressure. The precipitate was collected, dissolved in 30 mL diethyl ether and extracted with 30 mL of distilled H2O in a separatory funnel to remove the water-soluble impurities. MgSO4 was then used to dry the organic phase. The solvent was evaporated by a rotary evaporator and the residue was recrystallized from absolute ethanol solution. The crystalline product was washed with cold ethanol. Yield: 1.94 g (70%). Elem. Anal. Calcd. For C17H15N3O: C, 73.63; H, 5.45; N, 15.15. Found: C, 73.45; H, 5.35; N, 15.20. UV–Vis in dichloromethane: kmax(nm) (emax/M1cm1): 247 (8.83  104), 275 (8.89  104), 354 (8.90  104). IR: m(C@N) 1606, m(C„C) 2095, m(C@O) 1667 cm1. 1H NMR (CDCl3, d ppm):7.3 (2H, t), 9.03 (1H, s, NH), 7.44 (1H, s, NH), 6.987 (6H, m, H4, H3, H1), 6.98 (1H, t, H5), 6.63 (2H, d, H3), 3.47 (1H, s, acetylene), 2.53 (3H, s, COCH3) ppm, m.p. is 104–106 °C. trans-[Ru(bpy)(L1)Cl2] (1) Ruthenium trichloride (0.266 g, 1 mmol) and (0.253 g, 1 mmol) of the ligand (H2L) were dissolved in 70 mL of absolute ethanol. The mixture was refluxed for 1 h after which bpy (0.156 g, 1 mmol) was added to the solution. After allowing the reaction mixture to reflux for an additional 2 h, the solution was evaporated to dryness by a rotary evaporator. The crude product was dissolved in 20 mL dichloromethane and purified by chromatography on a 50 cm by 3 cm diameter column containing 250 g silica gel. The first red band which is trans-[Ru(bpy)(L2)Cl2] (2) complex was eluted by a mixture of (2:1) acetone/hexane and evaporated for dryness (vide infra). The second band which is trans-[Ru(bpy)(L1)Cl2] (1) was eluted by a acetone. Crystals were grown by a slow diffusion of diethyl ether into a solution of the complex in dichloromethane. Yield: 0.173 g (28%). Elem. Anal. Calcd. For C27H23Cl2N5O2Ru: C, 52.18; H, 3.73; N, 11.41. Found: C, 52.24; H, 3.68; N, 11.32. UV–Vis in CH2Cl2 [kmax/nm (emax/M1 cm1)]: 247 (25.49  03), 283 (26.336  03), 318 (13.787  03), 382 (7.657  103), 514 (5.799  103). IR (KBr, cm1): 1452 (v N@N), 1606 (v C@N), 1683 (v C@O), 1706 (v C@O). 1H NMR (300 MHz, CDCl3): 8.11 (4H, m, H1, H2), 8.09 (1H, d, H13), 7.97 (1H, d, H6), 7.86 (1H, t, H7), 7.84

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(1H, t, H12), 7.84 (1H, t, H8), 7.72 (1H, t, H11), 7.62 (2H, d, H3), 7.55 (2H, t, H4), 7.17 (1H, d, H10), 7.01 (2H, m, H5, H9), 2.85 (3H, s, COCH3), 2.74 (3H, s, COCH3). 13C NMR (300 MHz, CDCl3): 197.26 (C@O), 189.18 (C@O), 168.4, 158.9, 157.0, 156.8, 155.8 (C10), 155.4, 153.08 (C9), 139.4 (C11), 138.4 (C5), 153.4, 129.9 (C8), 129.2 (C1), 128.9 (C4), 124.8 (C12),124.2 (C7), 123.7 (C2), 122.7 (C3), 121.5 (C6), 121.2 (C13), 30.2 (CH3), 26.9 (CH3). trans-[Ru(bpy)(L2)Cl2] (2) Yield: 0.14 mg (22%). C27H23Cl2N5O2Ru: C, 52.18; H, 3.73; N, 11.41. Found: C, 52.31; H, 3.78; N, 11.44. UV–Vis in CH2Cl2 247 (25.49  03), 282 (26.836  03), 280 (26.336  03), 325 (12.787  03), 382 (7.057  103), 512 (5.299  103). IR (KBr, cm1): 1454 (v N@N), 1606 (v C@N), 1706 (v C@O). 1H NMR (300 MHz, CDCl3): 8.07 (2H, d, H3), 8.00 (1H, d, H13), 7.97 (1H, d, H6), 7.96 (1H, t, H7), 7.88 (1H, t, H12), 7.84 (1H, t, H8), 7.78 (2H, t, H4), 7.76 (1H, d, H9), 7.55 (4H, m, H1, H2), 7.14 (1H, d, H10), 7.05 (1H, t, H5), 7.05 (1H, t, H11), 5.93 (1H, s, @CHa), 5.67 (1H, s, @CHb),4.70 (C(OH)@C), 2.84 (3H, s, COCH3). 13C NMR (300 MHz, CDCl3): 189.3, 168.9, 158.8, 157.0, 156.7, 155.76 (C10), 153.4 (C9), 151.7, 139.5, 139.3 (C11), 138.3 (C5), 135.7 (C(OH)@C, 129.8 (C8), 128.9 (C1), 126.9 (C4), 124.7 (C12), 124.1(C7), 123.7 (C2), 122.48 (C3), 121.44 (C6), 121.05 (C13), 113.2 (@CHbHa), 30.3, COCH3. Instrumentation All 1D (1H, 13C, and DEPT-135, DEPT-90) and 2D (1H–1H COSY and C–1H) correlation spectroscopy NMR measurements were performed using a Bruker Avance 300 spectrometer. All chemical shifts were reported in ppm downfield of TMS and referenced using the chemical shifts of residual solvent resonances. IR spectra were measured by an FT-IR JASCO model 420. Elemental analyses were carried out on a Eurovector E.A.3000 instrument using copper sample13

tubes. UV–Vis/NIR spectra were recorded on a TIDAS fiberoptic diode array spectrometer (combined MCS UV/NIR and PGS NIR instrumentation) from j&m in HELLMA quartz cuvettes with 0.1 cm optical path lengths. Electrochemical measurements were performed in dichloromethane (Aldrich, HPLC grade) using BAS CV-27. All electrochemical experiments were done in a home-built cylindrical vacuum-tight one-compartment cell. A spiral-shaped Pt wire and an Ag wire as the counter and thin pseudo-reference electrodes were sealed into glass capillaries via standard joints and fixed by Quickfit screws. A platinum electrode was introduced as the working electrode through the top central port via a Teflon screw cap with a suitable fitting. It was polished with first 1 lm and then 0.25 lm diamond pasted before measurements. The cell was attached to a conventional Schlenk line via a side arm equipped with a Teflon screw valve and allowed experiments to be performed under argon atmosphere with approximately 5 mL of complexes solution. Tetrabutylammonium hexafurophosphate (0.1 M) was recrystallized twice and vacuum dried at 120 °C, and used as the supporting electrolyte. The temperature was controlled (at 25.0 ± 0.1 °C) by a Haake D8-G refrigerator. Referencing was done with an addition of one crystal of decamethylferrocene (Cp*2Fe) as an internal standard. Representative sets of scans were repeated with the added standard. A final referencing was done against the ferrocene/ferrocenium (Cp2Fe0/+) couple with E½ Cp*2Fe0/+ = 542 mV vs. Cp2Fe0/+ [33,34]. Spectroelectrochemistry of a representative complex 1 was performed by using an optically transparent thin layer electrode (OTTLE) cell [35]. OTTLE cell was home-made and equipped with Pt working and counter electrodes and a thin silver wire as a pseudo-reference electrode sandwiched between two CaF2 windows of a conventional liquid IR cell. The working electrode was positioned in the center of the spectrometer beam. The potential was controlled by the same BAS CV-27 that was used for cyclic voltammetry. At any given potential, the system was allowed to come to equilibrium (i  0 A) before the spectrum was taken.

Table 1 Crystal data and structure refinement for ligand and complex 1.

Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions

Volume Z Density (calculated) Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 26.30° Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole R1 =

P

||Fo|  |Fc||/

Ligand H2L

Complex 1

C17H15N3O 277.32 293(2) K 0.71073 Å Orthorhombic Pbca a = 12.0326(6) Å a = 90° b = 13.2499(7) Å b = 90° c = 19.1448(9) Å c = 90° 3052.3(3) Å3 8 1.207 Mg/m3 0.078 mm1 1168 0.30  0.25  0.15 mm3 3.12–26.30° 7 6 h 6 15, 16 6 k 6 10, 23 6 l 6 12 8623 3092 [R(int) = 0.0347] 99.9% Semi-empirical from equivalents 1.00000 and 0.86799 Full-matrix least-squares on F2 3092/0/194 1.047 R1 = 0.0590, wR2 = 0.1342 R1 = 0.1102, wR2 = 0.1558 0.268 and 0.273 e Å3

C27H23Cl2N5O2Ru 621.47 293(2) K 0.71073 Å Monoclinic P 1 21/n 1 a = 13.0700(6) Å a = 90° b = 15.0804(6) Å b = 105.762(5)° c = 13.6634(7) Å c = 90° 2591.8(2) Å3 4 1.593 Mg/m3 0.846 mm1 1256 0.35  0.25  0.1 mm3 3.10–26.32° 16 6 h 6 16, 18 6 k 6 18, 11 6 l 6 17 12,196 5267 [R(int) = 0.0287] 99.8% Semi-empirical from equivalents 1.00000 and 0.76999 Full-matrix least-squares on F2 5267/0/334 1.049 R1 = 0.0407, wR2 = 0.0852 R1 = 0.0679, wR2 = 0.0942 0.867 and 0.460 e Å3

P P P |Fo|; wR2 = { [w(F2o  F2c )2]/ [w(F2o)2]}1/2.

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O N N H

Cl

O

N(C2H5)3

+ H2N

N N H

N H

1a

( H2L

= R

H )

Scheme 1. Synthesis of H2L.

Fig. 1. (a) FT-IR spectra (a) H2L ligand after 0 and 30 min in absolute ethanol and (b) H2L ligand after 10 min in absolute ethanol.

Computational methods Full geometry optimization of 1 was carried out using density functional theory (DFT) at the B3LYP level [36]. All calculations were carried out using the GAUSSIAN 03 program package with the aid of the GaussView visualization program [37]. For C, H, Cl, N and O, the 6-31G(d) basis set were assigned, while for Ru, the MWB basis set with effective core potential was employed [38]. Vertical electronic excitations based on B3LYP optimized geometries were computed using the time-dependent density functional theory (TD-DFT) formalism [39–42] in dichloromethane using conductor-like polarizable continuum model (CPCM) [39–42]. Gauss Sum was used to calculate the fractional contributions of various groups to each molecular orbital [43].

good yield (Scheme 1). The pure product was obtained as yellow crystals by recrystallization from ethanol and it was characterized by 1H NMR spectroscopy, elemental analyses and X-ray diffraction. The organic compound acts as a bidentate NN0 ligand. The infrared spectrum of the H2L was shown in Fig. 1. The absorption at 3240–3139 cm1 was attributed to the m(NAH) vibration mode. A weak absorption at 2098 cm1 was assigned to C„C, very strong bands at 1667 cm1 were assigned to m(C@O) and absorptions at 1606 cm1 were attributed to m (C@N). The existence of one structural isomer for H2L ligand was confirmed by the presence of only one signal for each hydrogen in 1H NMR and 13C NMR spectrum. The 1H NMR showed two singlet signals assignable for two NH groups in the regions d 9.03 and 7.44 ppm. The acetylene and acetyl signals were found at 3.47 and 2.53 ppm respectively. The signals of C@O and C„C were observed

Crystallography A suitable crystals of either the ligand H2L or complex 1 was mounted on a glass fiber. The diffraction data sets were collected at room temperature employing enhanced Mo radiation, k = 0.71073 Å, using Xcalibur/Oxford Diffractometer equipped with Eos CCD detector. CrysAlisPro software was used for data collection, absorption correction and data reduction to give hkl files [44]. The structures were solved using SHELXTL program package [45]. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined using a riding model. Details of data collection and refinement are given in Table 1.

R

ethanol

H2L

R

H

O L1

Scheme 2. Catalytic hydration of the ligand H2L to L1 and L2.

Results and discussion

RuCl3

H2L

bpy

3

O2

N

Cl 13

1

N

N

5

N

N

+

6

O

N N O

Cl

Ru

12

Treatment of 1-(phenylhydrazono)-2-propanone (1a) with 4-ethynylamine and triethylamine in refluxing ethanol afforded the new ligand (1Z)-N-(4-ethynylphenyl)-2-oxo-N0 -phenylpropanehydrazonamide (H2L = PhNHN@C(COCH3)NHPhAC„CH) in a

HO

H

L2

4

Synthesis

R

RuCl3.nH2O

H

Cl

N Ru

N

Cl N

7 11

10

9

1

8

2

Scheme 3. Synthesis of [Ru(bpy)(Y)Cl2] (1–2).

Hb

Ha OH

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at 162.81 ppm and 162.26 ppm in the 13C NMR spectrum, respectively. The terminal acetylene group (C„CH) of the ligand (H2L) was catalytic hydrated via Markovnikov addition to produce enol ((C(OH)@CH2) within 10 min (Fig. 1). Water of RuCl33H2O presumably acted as the hydration source. The terminal acetylene group did not hydrolyzed in refluxing the ligand for 1 h in 90% ethanol in absence of the RuCl33H2O. The enol form then quickly rearranges to form a carbonyl group (COCH3) during the reaction (Scheme 2). The proposed mechanism (Scheme 2) of the

Ru(III)-catalyzed Markovnikov addition of water to alkynes is explained by steric and electronic arguments [46–50]. The better stabilization of a partial positive charge at the internal acetylene carbon (larger r-donor effect of R vs. H) as well as the steric repulsion between metal ion and alkyl group R favor an attack at the inner alkyne carbon atom [51,52]. The catalytic hydrated ligands H2L1 (keto form) and H2L2 (enol) are oxidized to azoimine (L1 and L2) by Ru(III) under the conditions of the synthesis. For the H2L ligand, the NAH protons appear as two singlets around 8.5 and 7.2–7.6 ppm, and these signals are

Fig. 2. 2D HMQC spectrum for complex 2 in CDCl3.

% Transmitance

100

1 2

98

96

94

1400

1600

1800 -1

Energy (cm ) Fig. 3. FT-IR spectra of complex 1 and 2.

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833

Fig. 4. ORTEP of H2L. Thermal ellipsoids of ORTEP plots are reported at 30% probability.

absent in all of the complexes, indicating the full oxidation of NH to azo and imine groups and the coordination of these nitrogens to the metal center. The ligands L1 and L2 have neither been isolated nor characterized spectroscopically. Two mixed-ligand ruthenium complexes having a general formula, [Ru(bpy)(Y)Cl2] (1–2) (where Y = L1 (1) and Y = L2 (2), bpy is 2.20 -bipyrdine) were achieved by the stepwise addition of equimolar amounts of bpy ligand to reaction mixture (Scheme 3). These complexes are air stable as solids or in solution and are soluble in common organic solvents. Their structures were confirmed by 1D 1H NMR, 13C NMR (distortionless enhancement by polarization transfer (DEPT-135, DEPT-90) spectra, 2D NMR (COSEY, hetero-nuclear multiple quantum coherence experiments (HMQC) and Heteronuclear Multiple Bond Correlation (HMBC), elemental analysis and X-ray diffraction for complex 1. The aromatic region in the 1H NMR spectra of complexes 1 (Fig. S1) and 2 (Fig. S2) consists of several coupled multiplets due to the aromatic protons of the phenyl rings of the azoimine and 2,20 -bipyridine ligand. In the aliphatic region, complex 1 (Fig. S1) showed two singlet at 2.85 and 2.74 which were assigned to the two CH3 of the two acetyl group. In DEPT-135 NMR (Fig. S3) and 13C NMR ((Fig. S4), complex 1 shows two carbonyl signals at 197.26, 189.18 ppm and two CH3 at 30, 26.9 ppm. The 1H NMR spectrum of 2 displayed two doublet resonances centered at d 5.93 and

6.67 ppm (Fig. S2) with a coupling constant of JH–H = 17.3 Hz assignable to the CHaHb group and one exchangeable singlet signals OH at 4.75 ppm. 13C NMR (Fig. S5), DEPT 135 (Fig. S6) and DEPT 90 (Fig. S7) showed that complex 2 has one signal for @CHaHb thirteen signals for Ar–H and six quaternary carbons (168.8, 157.0, 156.7, 151.7, 139.4 and 135.6 ppm) and one acetyl carbon at 189.3 which was assigned for the acetyl group attached to azoimine moiety. The carbon type (PhC(OH)@CHaHb)) of the enolate tautomer for complex 2 was determined by using distortionless enhancement by polarization transfer (DEPT-135) experiments (Fig. S6), and 2D hetero-nuclear multiple quantum coherence experiments (HMQC) (Fig. 2). The DEPT-135 spectrum showed only one negative signal at 113.2 which was assigned to PhC(OH)@CHaHb of the enol tautomer. Furthermore, The vinylic carbon of the enol tautomer PhC(OH)@CHaHb) was confirmed by 13C–1H correlation spectroscopy (HMQC) NMR (Fig. 2). There is a correlation between the vinylic carbon at 113.2 and their attached hydrogens (Ha and Hb) at 5.93 and 5.67, respectively. In HMBC spectrum (Fig. S8), there is a strong 1–2 correlation between the two hydrogens CHaHb and the enolate carbon (PhC(OH)@CHaHb) at 135.66 ppm and the Ar–H carbon (C1) (Scheme 3). The IR spectra of complexes 1 exhibited two sharp intense bands at 1683 and 1706 cm1 which corresponded to the two acetyl group. While complex 2 showed only intense band at

Fig. 5. ORTEP of 1. Thermal ellipsoids of ORTEP plots are reported at 30% probability.

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Table 2 Selected bond lengths and angles for the ligand H2L and complex 1. H2L

Complex 1

Bond lengths (Å) (experimental, optimized) N(3)AC(7) N(3)AC(10) O(1)AC(8) N(2)AC(7) N(2)AN(1) N(1)AC(1)

1.396(3) 1.403(3) 1.214(2) 1.289(3) 1.338(2) 1.400(3)

Ru(1)AN(1) Ru(1)AN(3) Ru(1)AN(5) Ru(1)AN(4) Ru(1)ACl(1) Ru(1)ACl(2) N(3)AC(7) N(1)AN(2) O(1)AC(8) O(2)AC(16)

1.940(2), 2.005(3), 2.116(3), 2.167(2), 2.361(9), 2.366(9), 1.320(4), 1.320(4), 1.198(5), 1.217(5),

Bond angles (°) (experimental, optimized) C(7)AN(3)AC(10) C(7)AN(3)AH(3A) C(15)AC(10)AN(3) C(11)AC(10)AN(3) C(7)AN(2)AN(1) N(2)AN(1)AC(1) N(2)AC(7)AN(3) N(2)AC(7)AC(8) N(3)AC(7)AC(8) C(6)AC(1)AC(2) C(6)AC(1)AN(1) C(2)AC(1)AN(1)

124.00(17) 118.00(16) 123.07(19) 118.25(19) 118.76(19) 118.17(18) 125.20(2) 116.00(2) 118.68(18) 120.20(2) 119.00(2) 120.90(2)

N(1)ARu(1)AN(3) N(1)ARu(1)AN(5) N(3)ARu(1)AN(5) N(1)ARu(1)AN(4) N(3)ARu(1)AN(4) N(5)ARu(1)AN(4) N(1)ARu(1)ACl(1) N(3)ARu(1)ACl(1) N(5)ARu(1)ACl(1) N(4)ARu(1)ACl(1) N(1)ARu(1)ACl(2) N(3)ARu(1)ACl(2) N(5)ARu(1)ACl(2) N(4)ARu(1)ACl(2) Cl(1)ARu(1)ACl(2)

75.70(11), 78.68 102.54(11), 100.38 177.18(10), 178.65 175.41(10), 177.35 105.86(11), 103.68 76.07(10), 74.35 96.48(8), 93.38 91.38(7), 90.08 86.63(7), 88.68 87.83(7), 89.65 91.44(8), 89.38 91.76(7), 89.49 90.48(7), 89.08 84.22(7), 86.68 171.99(3), 173.98

Table 3 CAH  p interactions parameters.

a

Compound

Ligand (H2L)

Complex 1

d H_cen (Å)a d\ (Å)b h (°)c

2.855 2.720 17.7

2.677 2.661 4.95

The distance between hydrogen atom and the centroid of the closest bond of the aromatic system. b The perpendicular distance between the hydrogen atom and the plane of the aromatic system. c The angle between d H_cen and d\.

2.002 2.065 2.118 2.159 2.487 2.503 1.337 1.333 1.261 1.252

1706 for the acetyl group at (Fig. 3). Both complexes have two bands around 1452 and 1606 cm1 which are assigned to N@N and C@N stretching bands, respectively.

Crystal structure Crystal structure data for the ligand H2L and trans[Ru(bpy)(L1)Cl2] (1) are compiled in Table 1 and ORTEP drawing of the ligand H2L and complex 1 are shown in Figs. 4 and 5,

Fig. 6. UV–Vis spectrum for 1 in dichloromethane. Inset shows simulated absorption spectrum of complex 1. (Black line) based on TD-DFT calculations, compared to excitation energies and oscillator strengths.

M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839 Table 4 DFT energies and % composition of selected occupied molecular orbitals and virtuals of complex 1 expressed in terms of composing fragments. Complex

Molecular orbitals

Energy (eV)

Ru

Cl

Azo

1

LUMO + 4 LUMO + 3 LUMO + 2 LUMO + 1 LUMO HOMO HOMO  1 HOMO  2 HOMO  3 HOMO  4

1.52 1.8 1.9 2.31 3.58 5.77 6.27 6.48 6.78 6.94

55 2 9 5 22 65 39 57 18 0

24 0 3 0 3 24 39 1 4 1

14 96 86 4 67 8 20 39 72 99

Bipy 6 2 2 91 8 4 2 3 6 0

1+

a-spin LUMO + 5 LUMO + 4 LUMO + 3 LUMO + 2 LUMO + 1 LUMO HOMO HOMO  1 HOMO  2 HOMO  3 HOMO  4 HOMO  5

2.12 2.62 2.77 3.08 3.68 4.96 7.31 7.4 7.66 7.82 7.82 7.93

1 32 1 2 11 52 0 3 2 1 11 0

88 14 1 96 2 6 0 4 71 4 3 22

10 54 98 1 85 7 100 93 27 92 69 75

0 0 0 0 2 35 0 1 1 3 17 3

b-spin LUMO + 4 LUMO + 3 LUMO + 2 LUMO + 1 LUMO HOMO HOMO  1 HOMO  2 HOMO  3 HOMO  4

2.73 3.06 3.38 4.91 5.31 7.31 7.4 7.63 7.69 7.82

2 3 55 17 72 0 3 15 14 0

0 0 31 3 16 0 1 22 21 1

96 1 8 78 8 100 93 27 27 93

2 96 6 3 4 0 3 36 38 6

respectively. The ligand H2L crystallized in the orthorhombic system with one molecule of the H2L per asymmetric unit. In all

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structures the NNC(COCH3)N (Fig. 4) backbone is almost planar. The bond length for N1AN2 is 1.338(2) Å. The bond length for N(2)AC(7) is 1.266 Å which is shorter than N(3)AC(7) 1.396(3) Å, the shortening support that the ligand is in a reduced form H2L. For complex 1, the structure shows (Fig. 5) that the L1 is coordinated to ruthenium, via dissociation of two acidic protons, as a bidentate N,N0 -donor. The Ru(II) ion of 1 occupies a pseudo octahedral coordination sphere made up of two trans chloride ligands and four nitrogen donor atoms (Table 2). In the equatorial plane, the five member rings described by the coordination of L1 and bpy have equivalent coordination bite-angles (N(4)ARu(1)AN(5), 76.07(10)° and N(1)ARu(1)AN(3), 75.70(11), respectively. However, it is interesting to note that the bond lengths between the azoimine nitrogens and Ru(II), (Ru(1)AN(1), 1.940(2) Å and Ru(1)AN(3), 2.005(3) Å), are significantly shorter than that of the bpy nitrogens to Ru(II), (Ru(1)AN(4), 2.167(2)Å and Ru(2)AN(5), 2.116(3)Å). The difference in RuAN bond lengths indicated that the MAL p interaction is localized in the MAAzo fragment [26–32]. However, it is noteworthy that the bond lengths RuAN(Azo) for complex 1 is slightly shorter than the corresponding length for similar reported ruthenium azoimine bidentate complexes, trans-[Ru(Az)(bpy)Cl2] (RuAN(azo) = 1.965(3) Å [53]). The shortening for RuAN(Azo) suggest that the ligand L1 is better p-acceptor ligand comparing to the previously prepared azoimine bidentate ligand [53]. The supramolecular structures of the free ligand and 1 are developed based on the CAH  p and hydrogen bonding interactions. The data summarizing CAH  p interactions are listed in Table 3. The h angle is relatively small, this indicates that the H  cent is close to be perpendicular to the plane of the aromatic system (Figs. S9 and S10). CAH  p interactions have been shown to play crucial role in the self-assembly of many crystalline chemical compounds [55–57]. The C(acetylene)AH  p interactions connect the molecular units of the free ligand to form chain structures run parallel to the a-axis (Fig. S9A). These chains are linked via the NAH  O hydrogen bonding interaction to form layer structures lie in the ab planes (Fig. S9B). The hydrogen bonding parameters are

Fig. 7. Isodensity plots of the HOMOs and LUMOs, orbitals of 1.

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M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839

Table 5 Computed excitation energies (nm), electronic transition configurations and oscillator strengths (f) for the optical transitions in the visible region of complex 1 and its corresponding cation 1+, transitions with f P 0.02 are listed). Complex

Wave length (nm)

Oscillator strength

Major contributions

1

565.4 522.6 438.3 422.3 406.7 392.4 389.3 371.7 365.1 346.2 343.4 334.6 327.9 323.2 295.8 288.9

0.0768 0.0432 0.0758 0.053 0.0215 0.0255 0.0308 0.0518 0.0207 0.0767 0.0724 0.0405 0.0583 0.0523 0.0916 0.0782

HOMO  2 ? LUMO (29%), HOMO  1 ? LUMO (65%) HOMO  3 ? LUMO (74%) HOMO  6 ? LUMO (18%), HOMO  5 ? LUMO (63%) HOMO  9 ? LUMO (25%), HOMO  6 ? LUMO (30%) HOMO  9 ? LUMO (28%), HOMO  8 ? LUMO (51%) HOMO  11 ? LUMO (41%), HOMO ? LUMO + 3 (18%) HOMO  11 ? LUMO (38%), HOMO  1 ? LUMO + 1 (23%) HOMO  12 ? LUMO (37%) HOMO  2 ? LUMO + 1 (18%), HOMO  2 ? LUMO + 4 (18%) HOMO  14 ? LUMO (18%), HOMO ? LUMO + 5 (60%) HOMO  14 ? LUMO (22%), HOMO ? LUMO + 5 (34%) HOMO  2 ? LUMO + 2 (17%), HOMO  1 ? LUMO + 2 (26%) HOMO  1 ? LUMO + 3 (23%), HOMO ? LUMO + 6 (40%) HOMO  1 ? LUMO + 3 (25%) HOMO  5 ? LUMO + 1 (63%) HOMO  4 ? LUMO + 1 (30%)

1+

726.3 594.4 590.0 571.9 561.1 517.9 497.3 489.4 483.1

0.02 0.0135 0.0119 0.0109 0.02 0.0241 0.0431 0.0868 0.0332

HOMO  1(a) ? LUMO(a) (35%), HOMO  1(b) ? LUMO + 1(b) (58%) HOMO  8(b) ? LUMO(b) (55%) HOMO  7(a) ? LUMO(a) (30%), HOMO  2(a) ? LUMO(a) (35%) HOMO  7(a) ? LUMO(a) (33%), HOMO  3(a) ? LUMO(a) (33%) HOMO  10(b) ? LUMO(b) (66%) HOMO  8(a) ? LUMO(a) (31%), HOMO  5(a) ? LUMO(a) (40%) HOMO  8(b) ? LUMO + 1(b) (66%) HOMO  8(a) ? LUMO(a) (45%), HOMO  5(b) ? LUMO + 1(b) (21%) HOMO  13(b) ? LUMO(b) (76%)

2.268 Å, 2.921 Å and 132.7° for H  O, N  O and NAH  O, respectively. Similarly, the C(phenylic)AH  p interactions link the crystalline units of the complex to form a chain structure in 101 direction (Fig. 9) [54,55]. Subsequently, the non-classical CAH  Cl hydrogen bonding interactions connect these chains to form the three-dimensional final structure. Electronic spectra and DFT calculation The UV–Vis spectrum of complex 1 has been performed in dichloromethane and depicted in Fig. 6. The complexes exhibit an intense transition in the UV region (220–320 nm), accompanied by a moderately intense band at longer wavelength (320–450 nm and low energy band at 512 nm. DFT calculations were carried out to explain the electronic structures of complexes 1 using GAUSSIAN 03 program. The singlet state geometry of the complex 1 in the ground state was optimized. The structural agreement has been observed from the comparison of bond distances and angles between calculated and X-ray determined structure (Table 2). Relative percentages of atomic contributions to the lowest unoccupied and highest occupied molecular orbitals have been placed in Table 4. Moreover, the isodensity plots for the HOMOs and LUMOs orbitals are shown in Fig. 7. Computation of 40 excited states of complex 1 allowed the interpretation of the experimental spectra for the complexes in the 300–800 nm range (Table 5). The calculated energy of excitation states and transition oscillator strength (f) are shown in Table 5. The absorption spectrum of 1 was simulated using Gaussian Sum software [43] based on the obtained TD-DFT results. Both the experimental UV–Vis spectrum of complex 1 reported in dichloromethane and its simulated absorption spectrum shown in Fig. 6 were in acceptable agreement. For complex 1, LUMO is constructed mainly from the p* orbital of azoimine with 22% metal dorbital character, which suggests significant back donation [56]. LUMO + 1 is constructed mainly from bpy while LUMO + 2 and LUMO + 3 are mainly azoimine in character. For the HOMOs, the three highest energy orbitals are Ru in character. These three HOMOs contain a sizeable contribution from azoimine and chloride ligands.

On the basis of its intensity and position, the lowest energy band at 512 nm (565.4 nm (calculated)) (Table 5) is resulted from HOMO  1 and HOMO  2 which have a sizable contributions of Ru(dp) orbitals and chloride to LUMO which has a significant contribution from the p* orbital of azoimine (L1). Thus this band is assigned to a mixture of intraligand (ILCT) and metal to ligand charge transfer (MLCT) transitions. The band at the moderate energy around 380 nm (438.3 nm, (calculated)) is resulted from HOMO  6 and HOMO  5 which have sizable contributions from bpy to LUMO, thus this band is assigned to intraligand (ILCT) (bpy (p)) ? L1 (p*)). The high intensity and energy band at 284 nm (293 nm (calculated)) resulted from HOMO  5 to LUMO + 2 thus this band is assigned as a ligand-to-ligand charge transfer LLCT (p–p* (phenyl ring) and n–p* (azomethine (C@N)) transitions. The transition at 280 nm (345 nm (calculated)) is resulted from the HOMO  14 ? LUMO and HOMO ? LUMO + 5 orbitals thus this bands is resulted from a mixture of metal–ligand charge transfer MLLCT (Ru(dp)) ? bpy(p*)) and ligand–ligand charge transfer LLCT (L1 (azo) ? L1(p*)). The band centered at 250 nm (290 nm (calculated)) is resulted from HOMO  4 and HOMO  5 to LUMO + 1, thus this band is assigned to ligandto-ligand charge transfer (L1(p)) ? bpy(p*)). Spectroelectrochemistry was performed on the complex 1 in dichloromethane in order to obtain the complex’s electronic absorption spectra in Ru(II) and Ru(III) oxidation states (Fig. 8). The oxidation of 1 is reversible with greater than 95% recovery of the Ru-(II) complexes spectrum. Theoretical calculations were performed on 1+; the calculations utilize both a and b occupied molecular orbitals. Relative percentages of atomic contributions to the lowest unoccupied and highest occupied molecular orbitals have been placed in Table 4. Moreover, the isodensity plots for the HOMOs and LUMOs orbitals are shown in Fig. 9. Ruthenium (III) contributes to the LUMO with a- and b-spin. HOMO to HOMO  5 and LUMO + 1 to LUMO + 4 of a-spin and HOMO to HOMO  1, LUMO + 1, LUMO + 4 of b-spin to are mainly azoimine in character. Chloride mainly contributes HOMO  8, HOMO  9, HOMO  10 of b-spin and HOMO  6, HOMO  7, HOMO  9, HOMO  10 of a-spin.

M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839

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25000

ε (M -1.cm -1)

20000

15000

10000

5000

0 200

300

400

500

600

700

800

W avelength (nm) Fig. 8. The UV–Vis spectroelectrochemical oxidation of 1 in dichloromethane (0.1 M TBAHF), 25 °C). Inset shows simulated absorption spectrum of complex 1+. (Black line) based on TD-DFT calculations, compared to excitation energies and oscillator strengths.

HOMO HOMO-1

HOMO-2

LUMO+1 β-molecular orbital

LUMO+2

HOMO

HOMO-1

HOMO-2

LUMO

LUMO+1

LUMO+2

LUMO

Fig. 9. Isodensity plots of the HOMO and LUMO orbitals of 1+.

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M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839

Fig. 10. (a) Differential pulse voltammogram and (b) cyclic voltammogram of complex 1 vs. Cp2Fe0/+ (TBAHF, 0.1 M, dichloromethane, 25 °C). Inset shows the Ru(II/III) at different scans rate.

TD-DFT calculations (Fig. 8) shows that the band centered at kmax = 650 nm (726 nm (calculated)) (Table 5) resulted from HOMO  1(a) and HOMO  1(b) which are mainly azoimine in character to LUMO (a) and LUMO + 1(b) which have a significant contribution from Ru(III), thus this band is assigned to L1 (p) ? Ru(III) (dp) ligand to metal charge transfer (LMCT). There is a decrease in the intensity of the MLCT centered at 512 nm and the growing of new band at 450 nm. The band centered at 450 nm (590 nm (calculated)) resulted from the overlap of several transitions, HOMO  8(b), HOMO  7(a) which are mainly chloride in character to LUMO(b) and LUMO(a) which has a larger contribution of Ru(III), thus this band is assigned to chloride(Pp)) ? Ru(III)(dp) ligand-to-metal charge transfer (LMCT) transition. The band centered at 290 nm (497 nm (calculated)) is resulted from HOMO  8(b) and HOMO  5(b) which are mainly chloride and azoimine (L1) in character to LUMO + 1(b) thus this band assigned to ligand to ligand charge transfer transition (LMCT). Electrochemistry The electron-transfer behavior of the complexes in dichloromethane solution was examined by cyclic voltammetry. The Ru(III/II) couples were calculated from average of the E°1/2 values for the anodic and cathodic waves from cyclic voltammetry. Decamethylferrocene was added as an internal standard near the end of the experiment. Ru(III/II) couples was referenced against Cp2Fe0/+ and can be referenced to the NHE by adding 0.62 V [57]. As a representative example, the cyclic voltammogram for complex 1 is shown in Fig. 10b. Complexes 1 exhibited a reversible oxidative

response at 604 mV vs. Cp2Fe/Cp2Fe+, which has been assigned to Ru(III/II) oxidation. Since the lowest unoccupied molecular orbitals are LAH in character, The one electron ligand reduction wave at 1072 mV vs. Cp2Fe0/+ (cathodic wave peak maxima) are assigned to the one electron reduction of the Azo(0/1): azo group [9–15]. The one-electron oxidation–reduction nature has been established by differential pulse voltammetry (Fig. 10a). The Ru(III/II) couple for complex 1 was used to find the ligand electrochemical parameters (EL(L1) = 0.48) for the new ligand L1 by using the previously found (EL(bpy) = 0.259) [58], EL(Cl) = 0.24 [58]) and Lever method [58]. The parameter EL reflects the stabilizing effect of the ligands on the Ru(II) state and so the greater value of EL, the more positive the Ru(III/II) couple. Conclusions New azoimine ligand has terminal acetylene group, PhANHAN@C(COCH3)ANHAPh(C„CH) (L), was synthesized. The group acetylene was catalytic hydrated via Markovnikov addition to acetyl (L1) and enol (L2) by stepwise equimolar addition of RuCl33H2O in refluxing absolute ethanol. Two novel mononuclear mixed-ligand ruthenium complexes having general formula, trans[Ru(bpy)(Y)Cl2] (1–2) (where Y = L1 (1) and Y = L2 (2) were achieved by the stepwise addition of equimolar amounts of (L) and bpy ligands. The presence of the two tautomers was confirmed by 1H NMR and 13C NMR. The crystallography and spectroelectrochemistry of trans-[Ru(bpy)(L1)Cl2] show that L1 is a strong p-acceptor that coordinates as a bidentate ligand via imine and azo nitrogens. The electrochemical parameter (EL(L)) for the ligand

M. Al-Noaimi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 828–839

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